1. Field of the Invention
This invention relates to a secondary cell having at least one electrode made of a carbon material as an active substance and a nonaqueous electrolyte, and more particularly, to an improved nonaqueous electrolyte secondary cell comprising an active substance made of a highly crystalline carbon material and, thus, having a high capacity, a high efficiency and improved low temperature characteristics.
2. Description of the Prior Art
Recently, attention has been paid to nonaqueous electrolyte secondary cells including lithium secondary cells for use as batteries of portable electric appliances and electric cars because of their high energy density and high working voltages. Some of these appliances may be used in winter or in cold districts, so that there is a demand of nonaqueous electrolyte secondary cells whose capacity and efficiency do not lower at low temperatures.
Carbon materials such as coke, graphite, graphitized carbon and the like have been proposed as active substance for electrodes of nonaqueous electrolyte secondary cells. Of these, a highly crystalline carbon material such as graphite or graphitized carbon is one of the materials which has attracted the greatest attention because of its high capacity and flat discharge potential.
On the other hand, studies have been made on a high dielectric constant solvent for nonaqueous electrolytes, and, as a result, it has been found that organic solvents such as propylene carbonate (PC) and ethylene carbonate (EC) are suitable for attaining high capacity. However, some types of solvents are very reactive with highly crystalline carbon materials and may react after repetition of charge and discharge cycles. Thus, these solvents cannot stand use as they are. Especially, propylene carbonate is very liable to undergo decomposition on the surfaces of an electrode along with the generation of gases. This eventually leads to lowerings of capacity and efficiency, and thus, cycle characteristics become poor. Ethylene carbonate has been investigated for use as a stable, high dielectric constant solvent, but because of its high solidifying point, the solvent is prone to rise in viscosity and is apt to settle when placed under low temperature conditions. Moreover, when ethylene carbonate is used in combination with low viscosity solvents such as tetrahydrofuran, 1,2-dimethoxyethane, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate and the like, satisfactory low temperature characteristics cannot be obtained.
Now, an attempt has been made, as set in Japanese Laid-open Patent Application No. 6-119939, to provide a mixed solvent prepared by mixing polypropylene carbonate and ethylene carbonate. As set out above, an electrolyte comprising propylene carbonate undergoes decomposition on repetition of charge and discharge cycles. Thus, it is not possible to attain a high discharge capacity.
Attempts have also been made not only to improve electrolytes, but also to improve a highly crystalline carbon material used as an active substance. For instance, Japanese Laid-open Patent Application No. 5-121066 proposes coverage of highly crystalline carbon particles each with a carbon material of low crystallinity. In order to avoid lowerings of capacity and efficiency, it is necessary that a carbon material of low crystallinity be used for the coverage in amounts as small as possible. However, it is very difficult to completely cover individual highly crystalline carbon particles with a thin layer of the low crystallinity carbon material according to the method set out in the above-mentioned Laid-open Application. More particularly, the method is not one which is simple and efficient from the industrial standpoint.
Japanese Laid-open Patent Application No. 6-132027 proposes an electrode which is obtained by mixing carbon particles serving as an active substance and a resin at the time of formation of an electrode and firing the mixture, thus having such a structure that the active carbon substance is combined with a carbonized product of the resin. A nonaqueous electrolyte secondary cell comprising this type of electrode is advantageous in that the carbonized product of the resin can suppress the reaction between the highly crystalline carbon and the electrolyte, thereby suppressing the decomposition of the electrolyte to an extent. However, the carbonized product is so small in amount and porous that it cannot completely cover individual particles of the highly crystalline carbon. In this sense, the decomposition of the electrolyte cannot be suppressed to a satisfactory extent.